The present invention relates to a method and a device for cleaning exhaust gas containing nitrogen oxides (NOx) from an internal combustion engine which includes at least one combustion space with an exhaust gas outlet, to which is connected an exhaust tract with an exhaust line and with at least one catalytic converter.
The combustion of hydrocarbons, such as occur, for example, in gasoline, with air in an internal combustion engine, in particular a diesel or gasoline engine, gives rise not only to carbon dioxide and steam as the main combustion products, but also to secondary products and pollutants. These are essentially hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx). The amount of these pollutants which the exhaust gases contain depends, in particular, on the air/fuel ratio which prevails in the internal combustion engine. Low air/fuel ratios are referred to as xe2x80x9crichxe2x80x9d mixture composition (air deficiency). A xe2x80x9cleanxe2x80x9d mixture composition (air excess) is due high air/fuel ratios.
It is known that, with an air deficiency, the exhaust gas contains a relatively large amount of CO and HC, whereas, in the case of an air excess, CO and HC may be oxidized almost completely. The NOx content passes through a maximum in the range of a slightly lean mixture composition. In this range, however, the specific consumption is also at an optimum for internal combustion engines, in particular for gasoline engines. If, therefore, for example gasoline engines are set at an optimally low consumption, there are high NOx concentrations, along with moderate CO and HC concentrations in the exhaust gas.
In particular, developments in traffic density in conurbations and metropoles have lead to the emission of the pollutants CO, HC and NOx being limited by law, first in the USA and later in Europe. The limits applicable at the present time, in particular for NOx, cannot be adhered to through the use of so-called primary measures, that is to say by optimizing the combustion process in order to avoid the formation of pollutants, in particular of thermal NOx. So-called secondary measures, in particular catalytic exhaust gas cleaning, are therefore employed in order to remain within these limits. Depending on the catalytic converter used, high conversion rates for all three of the above-mentioned pollutant components can be achieved in the case of a stoichiometric air/fuel ratio, but, in gasoline engines with lean mixtures and, in general, in diesel engines, the NOx fraction cannot be converted without additional measures.
For this purpose, it is known, for example from German Published, Non-Prosecuted Patent Application DE 40 03 515 A1, corresponding to U.S. Pat. No. 5,085,840, that, in order to reduce the NOx to N2, a reducing agent is introduced from a reducing agent reservoir through a reducing agent line into the exhaust tract and is intermixed with the exhaust gas flowing in the latter. The exhaust-gas/reducing-agent mixture is then subsequently supplied to a catalytic converter for the catalytic conversion of the NOx.
In this case, however, a stoichiometric ratio between NOx and reducing agent can be achieved only with difficulty because of fluctuating NOx concentrations in the exhaust gas: On one hand, continuous quantity metering of reducing agent at the level of NOx maxima which occur leads, below the NOx maxima, to an undesirable emission of reducing agent in the exhaust gas; on the other hand, a continuous quantity metering of reducing agent, for example at the level of an NOx average, leads, at the NOx concentrations in the exhaust gas which occur above this, to an undesirable emission of NOx.
In order to solve this problem, in particular to minimize the losses of reducing agent, catalytic converters have been developed, the coating of which can absorb to saturation, that is to say has a storage capacity for reducing agent and/or NOx. Using such a catalytic converter, the reducing agent can be introduced continuously in an average quantity into the exhaust tract. For example, overdoses of the reducing agent which occur are then stored in the catalytic converter and are discharged when NOx maxima occur later, and vice versa. However, for sufficient storage, this procedure necessitates a catalytic converter with a large storage volume.
It is accordingly an object of the invention to provide a method and a device for cleaning exhaust gas containing nitrogen oxide (NOx) from an internal combustion engine, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and which are effective, in particular, even when a catalytic converter has a small storage volume.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of cleaning exhaust gas containing nitrogen oxide from an internal combustion engine having at least one combustion space with an exhaust gas outlet, and an exhaust tract with an exhaust line connected to the exhaust gas outlet and with at least one catalytic converter, the method which comprises:
introducing reducing agent from a reducing agent reservoir through a reducing agent line into the exhaust tract and intermixing the reducing agent with the exhaust gas flowing in the exhaust tract to form an exhaust-gas/reducing-agent mixture, whereby the reducing agent is metered more than once to the exhaust gas in dependence on an amount of NOx generated in the internal combustion engine, for each emission of exhaust gas from each combustion space, by at least one metering device and a further control device and/or regulating device, and thereby adapting portions of the reducing agent to the amounts of NOx respectively generated; and
supplying the exhaust-gas/reducing-agent mixture to a catalytic converter for catalytically converting the NOx.
With the above and other objects in view there is also provided, in accordance with the invention, a device for cleaning exhaust gas of an internal combustion engine containing nitrogen oxide, wherein the engine has at least one combustion space with an exhaust gas outlet, and an exhaust line of an exhaust tract is connected to the exhaust gas outlet, comprising:
a reducing agent reservoir and a reducing agent line connecting the reducing agent reservoir to the exhaust tract;
at least one metering device for introducing a reducing agent from the reducing agent reservoir through the reducing agent line into the exhaust tract and for intermixing the reducing agent with an exhaust gas flowing in the exhaust tract;
a catalytic converter connected in the exhaust tract for receiving an exhaust-gas/reducing-agent mixture for a catalytic conversion of nitrogen oxide produced in the internal combustion engine; and
a control device connected to the metering device and configured to meter the reducing agent to the exhaust gas more than once by portion during each working stroke in dependence on a NOx content generated by the internal combustion engine.
In other words, the objects of the invention are satisfied in that the reducing agent is metered to the exhaust gas flowing in an exhaust tract, as a function of the generation of NOx by the internal combustion engine, through the use of a metering device and a further control device in synchronism with and/or out of phase with the respective emission of exhaust gas from a combustion space and in portions adapted to the NOx quantities generated, in particular in synchronism with and/or out of phase with the working stroke of the individual combustion spaces or in synchronism with and/or out of phase with a complete working stroke of the internal combustion engine. Such metering almost completely avoids overdoses of the reducing agent in the exhaust gas, as a result of which, for example, a stoichiometric ratio between NOx and reducing agent is achieved and cleaning exhaust gas containing NOx in an internal combustion engine is advantageously improved.
Moreover, the internal combustion engine preferably has an electronic engine control which, from data and measurement values available and/or arriving for controlling and/or regulating the internal combustion engine, also generates signals for controlling and/or regulating the metering device which are capable of being transmitted through the use of an electrical connection. The measurement values originate, for example, from measuring sensors which are disposed, for example, upstream and downstream of a catalytic converter located in the exhaust tract, the former transmitting, in particular, NOx and NH3 measurement values and the latter, in particular NHx, NH3 and NO measurement values to the electronic engine control. As a result, advantageously, in particular, ammonia slippage which may possibly occur and also a residual emission of NO can be detected and can be corrected through the use of the electronic engine control. The catalytic converter preferably used according to the invention for cleaning the exhaust gas therefore advantageously has to store only to the extent of control fluctuations of NOx and/or reducing agent, for which purpose a large storage volume is not necessary.
In accordance with a preferred feature of the invention, the reducing agent used is fluid ammonia which is carried in a reducing agent reservoir and is introduced, as required, into the exhaust tract. Alternatively, the reducing agent may also be carried as a stored precursor, for example urea, in the reducing agent reservoir and can be produced as required, in particular pyrolitically, and subsequently introduced as fluid into the exhaust tract.
The metering device, which has, in particular, at least one metering valve, is disposed preferably between the reducing agent line and the exhaust tract. Since the reducing agent is present under a constant pressure in the reducing agent line, the portions of reducing agent to be metered can advantageously be determined solely through the opening times of the metering device, or through the opening times of each metering valve. The opening times of the metering device are controlled and/or regulated through the use of the signals generated in the electronic engine control, as a function of the data and measurement values present in said engine control, in particular as a function of consumption in relation to a fuel supply. As a result, according to the invention, preferably, for example, the stoichiometic ratio required in each case between the pollutant component NOx and the reducing agent ammonia can be achieved in the exhaust gas.
The reducing agent is therefore introduced from the reducing agent reservoir, for example, through the reducing agent line and the metering valve. The constant pressure in the reducing agent line is generated by a pressure generation device, in particular a pump. The metering valves are advantageously constructed in such a way that they ensure a reliable intermixing of the reducing agent in the exhaust gas, for example through the use of atomizer nozzles as outflow orifices.
Particularly preferably according to the invention, the reducing agent is in each case introduced and intermixed with the exhaust gas at the exhaust gas outlet of each combustion space of the internal combustion engine. Through the reducing agent being introduced in this way in synchronism with and/or out of phase with the respective emission of exhaust gas from each combustion space, the losses of reducing agent can be minimized and the metering of reducing agent organized, as required, particularly also in the case of lean mixtures and taking into account NOx maxima which occur in the exhaust gas, since the control and/or regulating frequency of the metering valves needs to be adapted in each case to the cycle of a combustion space only in synchronism and/or out of phase.
Alternatively and/or additionally, a reducing agent is introduced into the exhaust line and intermixed with the exhaust gas jointly for the exhaust gas of all of the combustion spaces. The introduction of the reducing agent into the exhaust line is distinguished in that, in this case, to achieve an approximately stoichiometric ratio between reducing agent and nitrogen oxides (NOx), the ammonia can be introduced into the exhaust gas, in particular, with less outlay in terms of apparatus for this purpose, but with a higher control and/or regulating frequency of the at least one metering valve, said frequency being adapted to the cycles of all of the combustion spaces.
The quantity of ammonia required overall during a working stroke may, of course, be supplied in one or more metering operations distributed over the stroke, depending on requirements, for example as a function of rotational speed and load.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for cleaning exhaust gas containing nitrogen oxide (NOx) from an internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.